This invention relates to fish culture tanks, and particularly to a type of fish culture tank which is, to a large extent, self-cleaning.
Culture tanks may be used to grow fish from a small hatchery stage up to a larger stage. For example, salmon may be grown to the smolting stage, or trout may be grown to harvest size. The use of culture tanks has several advantages over raceways, in that they take up much less space, simplify the problems of managing the quality of various factors related to the water, such as dissolved oxygen content, and make it feasible to recycle the water in order to conserve water, simplify water waste treatment, and also to take advantage of conserved heat.
One of the main costs in fish rearing, whether in tanks or in raceways, has been the large labor costs involved in keeping the tanks clean or in cleaning them from time to time. Generally speaking, culture tanks, especially these of the present invention, are more easily cleaned than are raceways, but they have still been expensive to keep clean.
A prior method for cleaning tall, cylindrical, fish culture tanks used intermittent bottom-to-top high-flow flushing. This practice had several disadvantages. First, carrying the particulate matter up a vertical cylindrical tank and into an overflow drain involved imparting upward velocities of at least one meter per minute, considering the particle sizes normally encountered in fish culture tanks. Second, forcing the solid particles upwards through masses of fish (which may be in the density of 16 to 160 kilograms per cubic meter) tended to result in the solid waste particles being trapped in the gills of the fish. Third, these strong flows, even if used only 45 minutes every 6 hours, as was typical, excited and stressed the fish, leading to some mortality and slowing their growth. Fourth, high-flow pumping, which was in the order of 10,000 liters per minute for normal sizes of fish culture tanks, was expensive both in the initial cost of the pump needed and also for the operational expense of the pumping, once installed.
Another prior method for cleaning tall, cylindrical, fish culture tanks used a permanently installed rotating vacuum system. The vacuum system utilized an axially-extending vertical pipe with a generally horizontal radial extension at the bottom going out to the wall of the tank. At the distal end of the radial extension was a vacuum head. An airlift or a pump supplied the vacuum force. During cleaning, the axial pipe or its horizontal extension was rotated, and wiper blades on the horizontal extension pushed solid waste towards the tank wall, where it was sucked up by the vacuum head. This method was complicated, involved expensive equipment, was expensive in operation, cluttered the internal space of the tanks, and set up both moving and stationary obstructions that sometimes injured the fish.
Various methods have been used for cleaning shallow cylindrical tanks, such as manual vacuuming and lowering the water level while developing a central vortex, but these methods were not practical once the depth of the culture water in the tank exceeded more than about 5 feet. Tanks embodying the present invention can readily be made to accommodate depths of twenty or thirty feet or more of water.
The present invention has as one of its objects the provision of a self-cleaning tank that can enable the same staff to handle at least three times as many tanks or to take one-third the time in handling the tank, and thereby reduce the labor costs of cleaning up to about 85%.
Another object of the invention is to provide a system for gently removing the solid waste, regardless of the tank depth.
Another object of the invention is to provide an improved tank which can safely accommodate more fish than it could otherwise and can be used in a recycle system to provide improved growing conditions.
Another object of the invention is to provide a self-cleaning fish culture tank in which it becomes easier to remove the solids and dead fish found in the system, while at the same time saving the live fish that happen to be entrapped within the removal lines.